1. Field of the Invention
This invention relates to an optical communication system by which desired information is transmitted by way of a light beam which propagates in a space.
2. Description of the Prior Art
An optical communication system is conventionally known and disclosed, for example, in Japanese Patent Laid-Open Application No. 2-276328 and also in Japanese Patent Laid-Open Application No. 1-30573 wherein part of a beam of light having a predetermined polarization plane and sent out toward an object for transmission is turned back and measured together with light for measurement coming from the object for transmission so that the irradiating position of such beam of light can be confirmed readily.
An exemplary one of such conventional optical communication systems is shown in FIG. 19. Referring to FIG. 19, a conventional optical communication system shown is generally denoted at 1 and includes a laser diode 2 which is driven in accordance with a predetermined information signal to emit therefrom a light beam LA1 having a predetermined polarization plane.
The light beam LA1 is converted into a parallel light beam by a lens 4, passes through a polarizing prism 6 and is introduced to a half mirror 8.
The half mirror 8 passes part of the light beam LA1 therethrough, and such passing light is sent out toward an object for transmission by way of a pair of lenses 16 and 18.
In this manner, the optical communication system 1 sends out a light beam LA1 having a predetermined polarization plane toward an object for transmission.
Meanwhile, the half mirror 8 reflects the remaining part of the light beam LA1 from the polarizing prism 6 toward a corner cube prism 10 and passes and introduces turned back light reflected from the corner cube prism 10 to an image pickup element 14 by way of a lens 12.
In this manner, the optical communication system 1 separates part of a light beam LA1 to be sent out toward an object for transmission and turns back the optical path of the separated light so that the separated light may be focused upon the image pickup element 14.
On the other hand, the lens 18 receives a light beam LA2 coming from the object for transmission and introduces it to the polarizing prism 6 by way of the lens 16 and the half mirror 8. Here, the object for transmission is constructed such that it emits a light beam having a polarization plane perpendicular to the polarization plane of the light beam LA1.
The optical communication system 1 thus reflects the light beam LA2 by means of the polarizing prism 6 and then focuses the light beam LA2 upon a light receiving element 22 by means of a lens 20.
In this manner, the optical communication system 1 is constructed such that it receives a light beam LA2 coming from an object for transmission thereby to receive information carried on the light beam LA2.
The lens 18 also receives, together with such light beam LA2, light (which will be hereinafter referred to as observation light) L1 which advances from the background of the object for transmission toward the optical communication system 1. The thus received observation light L1 is introduced to the image pickup element 14 by way of the lens 16, half mirror 8 and lens 12.
Consequently, a component of the observation light L1 parallel to an optical axis of the light beam LA1 is introduced into the lens 12 in parallel to the reflected light from the corner cube prism 10. Accordingly, the reflected light from the corner cube prism 10 enters the lens 12 along such an optical path that it seems as if it is emitted from an irradiating position of the light beam LA1 toward the optical communication system 1.
Consequently, with the optical communication system 1, a pickup picture image forming a bright spot can be obtained at the irradiating position of the light beam LA1 on the image pickup element 14, and accordingly, the irradiating position of the light beam LA1 can be confirmed readily.
By the way, it is considered convenient if such optical communication system 1 can be reduced in size. In particular, the optical communication system 1 can be installed by a simple operation because the irradiating position of the light beam LA1 can be detected readily. Accordingly, the optical communication system 1 can be installed at a desired location and utilized, for example, for relaying of television broadcasting or the like in accordance with the necessity. However, in such an instance, the irradiating position of the light beam LA1 may possibly be displaced by vibrations or the like of the optical communication system 1.
One of possible solutions to this problem is to correct, using, for example, a servo technique, the irradiating position of the light beam LA1 with reference to the light beam LA2 coming from the object for transmission. However, the solution has a problem that, where the transmission distance is great, it is difficult to correct the irradiating position of such light beam LA1 with a high degree of accuracy due to an influence of reflected light reflected in the inside of the optical communication system 1.
Further, the solution has another drawback that, when a fog or the like gathers or sunlight or the like is inadvertently admitted into the optical communication system, the light beam LA1 is sometimes irradiated in a direction quite different from the direction toward an object for transmission, and after the fog lifts or when the sunlight is no more admitted into the optical communication system, the irradiating direction of the light beam LA1 must be re-adjusted so that servoing may be rendered effective.
In particular, if a fog gathers, then the intensity of the light beam LA2 is decreased extremely, and consequently, the output signal level of a light receiving element for detecting such light beam LA2 drops. Consequently, the SN ratio of the output signal is deteriorated, and the light beam LA1 may be irradiated in error in a direction quite different from the direction toward the object for transmission. On the other hand, when sunlight is admitted into the optical communication system, the irradiating position of the light beam LA1 will be corrected to the direction of the sun in error.